Networks typically include one or more nodes that are operable to transmit and receive messages. In a wireless network, for example, the RF output of a transmitter produces a broadcast region where signal reception is possible. Providing point-to-point information links is one of the basic problems associated with wireless communications. For example, as shown by FIG. 1a, receiver R1 and receiver R2 both are in the broadcast region of transmitter T3. The message to be communicated from transmitter T3 to receiver R1 may be different from the message to be communicated from transmitter T3 to receiver R2, thus requiring some form of signal isolation. One well-known solution to this problem relates to sending transmissions across different physical layers. For example, signals may be separated by frequency, time, or more generally, by waveforms that have low cross-correlation. Indeed, conventional communications technologies provide signal isolation using some form of orthogonal signaling.
There are many practical advantages associated with orthogonal signaling. Accordingly, alternatives rarely are considered, especially because non-orthogonal signaling may seem counterintuitive in some circumstances. Signaling in wireless networks may result in data collisions. For example, as networks become more complicated (e.g. by including multiple transmitters and receivers and allowing messages to travel across many paths at once), frames (which comprise the message to be communicated) collide or conflict. The heavier the communications volume, the worse the collision problems may become. The typical result is degradation of system efficiency, because when two frames collide the data contained in both frames usually is lost.
To coordinate wireless transmissions to nodes with overlapping wireless propagation paths, conventional networks have used contention arbitration protocols. Such protocols include, for example, the Aloha protocol. The Aloha protocol employs signals (sometimes referred to as beacons) that are sent at precise intervals, which indicate when the channel is clear to send a frame for each source. If a collision is expected, the transmitter may back off and try to send the frame later.
Numerous protocols have been developed to provide improvements and/or alternatives to the basic Aloha protocol and to solve the collision problem while increasing network throughput. For example, the CSMA protocol involves potentially each node in the network trying to predict whether a collision will occur. When collisions are predicted, transmitters stop sending, wait an amount of time (e.g. a random amount of time), and then try to transmit again. As another example, the TDMA protocol is based on the allocation of unique time slots over a single frequency to access a network, thus reducing the possibility of interference. In the FDMA protocol, the given bandwidth is divided into smaller frequency bands, or subdivisions. Each subdivision has its own carrier frequency, and a control mechanism is used to ensure that two or more earth stations do not transmit in the same subdivision at the same time, and thereby designate a receiver for each subdivisions. The OFDM protocol divides the frequency spectrum into subbands small enough so that channel effects are constant (e.g. flat) over a given subband. Then, a modulation is sent over the subband. When properly implemented, the fast changing effects of the channel (e.g. multipath) disappear, as they are made to occur during the transmission of a single symbol and are thus are treated as flat with fading at the receiver.
Unfortunately, while these techniques were designed to compensate for certain problems with traditional forms of network communications, they still suffer several drawbacks. For example, the protocols generally send a single message in any given transmission. Any node for which the transmission is not intended that receives the message simply ignores it. This may result in wasted time, frequency, and/or bandwidth. Additionally, while these protocols were designed to reduce the contention problem and to increase throughput, still further improvements are desirable. Thus, it will be appreciated that there is a need in the art to provide systems and/or methods that further reduce contention problems and increase throughput by, for example, allowing simultaneous transmissions to multiple nodes within a network.